The present invention relates to a device for tensioning knitted fabric in a circular knitting machine, in particular of the large-diameter type.
Devices for tensioning knitted fabric in large-diameter circular knitting machines are known, and comprise a plurality of tensioning rollers the axes whereof are arranged co-planar along a broken line, i.e. a polygonal, inscribed in a circumference which is concentric to the machine. The tensioning rollers are arranged below the knitting region of the needles and each roller is in contact with a presser roller; the knitted fabric formed by the needles is conveyed between the presser rollers and the tensioning rollers and these are rotated about their axis so as to apply a preset tension to the product while it is being formed.
In some types of device of this kind, for example of the type described in the published U.K. patent application No. 2.099.027A, the tensioning rollers are rotated individually by means of a lever connected to its related roller by means of a ratchet mechanism which allows said related roller to rotate in only one direction. The lever is actuated with reciprocating motion, and the advancement stroke, which is obtained by an element which rotates about the axis of the machine and progressively makes contact with all the levers, has no effect on the rotation of the related tensioning roller, since the ratchet mechanism operates freely, while the return stroke, which is obtained by means of a pneumatic actuator, rotates the related tensioning roller according to an angle of preset amplitude.
Though such devices succeed in tensioning the knitted fabric, they have some disadvantages. In particular, they are complicated in terms both of their manufacture and of their assembly, mostly as to the positioning of the pneumatic actuators. The levers actuating the tensioning rollers may furthermore accidentally catch in the fabric. Finally, the tension applied to the knitted fabric is not constant but intermittent due to the fact that during the advancement stroke of the lever the related tensioning roller does not move.
In other types of device, each tensioning roller has a shaft connected to the shaft of the contiguous rollers by means of conical gears, and a single electric motor capable of exerting torque even with zero rotation is used to serially rotate all the tensioning rollers. Such devices, though they have no elements protruding from the tensioning rollers, are extremely complicated from a mechanical point of view due to the conical-gear transmission between the rollers. With a transmission of this type, the device furthermore operates correctly only with a virtually perfect meshing of the conical gears, with adjustment problems during assembly and in maintenance. Furthermore, assuming all the rollers are tensioned at zero speed and that all the drops are excluded from the knitting, when they resume forming their row of knitting in sequence from one end to the other of the broken line along which the rollers are arranged, said new knitted rows are not tensioned adequately until fabric is generated along the entire broken line so that the commonly rotating rollers start to rotate and engage with the newly produced fabric.